## Fluids

#### Room Enclosure Steady State Thermal Air Volume vs. Convection

• miniseeker22
Subscriber

I am trying to simulate a vehicle enclosure that has a solar load, ac convection inside, and a shelf of different volumes containing heat generation.

In the first image, you can see my convection coefficient of 20 W/m2 at ambient of 30C which I was using to simulate AC conditions dominates the interior. I did apply it to all surfaces inside besides the ceiling. The solar loading is 1120 W/m2 to simulate the worst of the afternoon. The volumes on the shelving have a range of internal heat generation from ~51000 to ~2800 W/m3. My confusion here is why none of the volumes are showing variation in heat and why the temperature on the room is so outrageous. Maybe that's valid?

I reviewed some old boards here and saw it was suggested to do a volume extraction to simulate air. So I redid the model and created an air material at 25C. Then to fulfill the need of convection, I put a natural convection all around the outside walls. Now this is starting to have a lot more palatable temperatures, but it still seems off to have such high temperatures in the interior. Any suggestions?

My old sim professor always suggested to start off with a steady state model before jumping straight into fluid flow. So that's what I'm trying to do before adding complexity. On another side note, in order to get the volume extraction, I had to make a hole in my enclosure at the top where I had a previously extruded box for a faux AC. Does that affect my enclosure results? Is there a way to get that extraction without a hole?

• miniseeker22
Subscriber

• Rob
Ansys Employee

If the primary mechanism of heat transfer is convection why are you using a Mechanical (solids) solver and not CFD (fluids)?

• miniseeker22
Subscriber

Ok, so does the Mechanical solver have no redeemable results? What I was attempting here will never show reasonable results because it is not the primary mechanism of heat transfer, correct? If not, can you show me what was incorrect? Like I said, my professor told me this was a good route. Being an ansys professional, I will follow your advice and change my ways.

• Rob
Ansys Employee

Mechanical will be faster but you're using correlations for convective heat flow so accuracy may be an issue. Fluent will solve the fluid flow so get the convective heat transfer right, assuming you use a suitable mesh etc.  Both approaches can be used, but you need to decide what is important and what data is needed to choose the best one.

• miniseeker22
Subscriber

What is the range of error in terms of accuracy? Could it be <15%? It seems like the obvious error cause is how I inputted convection in the Mechanical version. Is there a better way or do you have any idea why my temperatures are so outrageous?

I am concurrently setting up a CFX model right now. My class only went through CFX, but I was told the transition to Fluent would be minimal...I just haven't had the time. As opposed to Mechanical which easily allowed internal heat generation in W/m3, CFX is only giving me a source unit option of W/m2. By applying this heat as a cross-sectional area the best comparison? Is there another way I'm not seeing?

• miniseeker22
Subscriber

disregard that last statement...I figured it out.

• Rob
Ansys Employee

The error will depend on the settings and how buoyant the flow it.

• miniseeker22
Subscriber

Hi rwoolhou,

Here are the results for my CFX based on the same inputs. These results show that at steady state, the room will be filled with 25C air, but some of the items on the shelf could still be 750C or higher? The lower wattages are fine on the bottom shelf, but the higher "processors" at 300 watts or so is still very high. Could this be true?

This makes me think that the boundaries aren't sharing thermal contact like they should although I have them enabled. I even enabled the appropriate internal heat generation at each interface (fluid fluid & fluid solid) under each domain (each item is a different domain. So I'm a bit confused why the air is a solid block for the most part besides the shell edges where there is impact from solar. I have an inlet coming at 25C from the ceiling at 15 m/s. When I call out the fluid domain to be 25C, can a boundary inlet source override it?

• miniseeker22
Subscriber

• miniseeker22
Subscriber

Here is a very specific example.

One of the lower boxes has its own domain and has thermal energy heat transfer in the solid models. I do not have an initialization here because it only allows temperature.

Under that box domain I have a fluid solid interface and a solid solid interface. For both of those I have a boundary source where I can say the energy total source is 25 W. Is this the correct way to do this? If I haven't mentioned something it has been untouched.

For the overall model fluid to solid interface, I have heat transfer checked and a thin material of aluminum.